Story highlights

The National Highway Traffic Safety Administration conducted tests this week

These found a greater fire danger involving the Volt's lithium-ion batteries

There is no recall, and the agency notes there have been no real-world incidents

GM has previously defended the car as safe

Federal safety regulators announced Friday that they have opened "a formal safety defect investigation" into the Chevrolet Volt over concerns the electric car's battery may pose a significant fire risk.

The National Highway Traffic Safety Administration has been looking at the car for months, explaining in a press release that this includes crash-testing the Volt in May and conducting three tests last week focused specifically on the vehicle's lithium-ion batteries.

"The agency is concerned that damage to the Volt's batteries as part of three tests that are explicitly designed to replicate real-world crash scenarios have resulted in fire," the administration said. "NHTSA is therefore opening a safety defect investigation of Chevy Volts, which could experience a battery-related fire following a crash."

The agency noted that there are no known "real-world" instances of battery problems causing fires to erupt after a driver crashes a Volt, and added that "Volt owners who have not been in a serious crash do not have reason for concern."

Nor has the federal regulator ordered any recall, though it did say that might occur "if NHTSA identifies an unreasonable risk of safety."

The agency is working with General Motors, plus the federal defense and energy departments, "to assess the causes and implications" of the fires, it said.

The agency reiterated guidelines for what drivers and first responders should do if an electric vehicle is involved in a crash, including taking precautions to avoid shocks and using "copious amounts of water if fire is present." No problems have been detected in other such cars.

"NHTSA testing on electric vehicles to date has not raised safety concerns about vehicles other than the Chevy Volt," the agency noted.

Earlier, GM spokesman Greg Martin said the automaker has been unable to duplicate the fires, even after subjecting the Volt battery pack to more than 300,000 safety tests.

The Volt passed other safety administration tests for protecting vehicle occupants, earning a five-star rating for overall safety, side impact and rollover risk and four stars for frontal crash protection.

"First and foremost, I want to make this very clear: The Volt is a safe car," Jim Federico, GM chief engineer for electric vehicles, said two weeks ago in a prepared statement. "We are working cooperatively with NHTSA as it completes its investigation."